Ehrhart clutters: Regularity and Max-Flow Min-Cut

If C is a clutter with n vertices and q edges whose clutter matrix has column vectors V={v1,...,vq}, we call C an Ehrhart clutter if {(v1,1),...,(vq,1)} is a Hilbert basis. Letting A(P) be the Ehrhart ring of P=conv(V), we are able to show that if A is the clutter matrix of a uniform, unmixed MFMC clutter C, then C is an Ehrhart clutter and in this case we provide sharp bounds on the Castelnuovo-Mumford regularity of A(P). Motivated by the Conforti-Cornuejols conjecture on packing problems, we conjecture that if C is both ideal and the clique clutter of a perfect graph, then C has the MFMC property. We prove this conjecture for Meyniel graphs, by showing that the clique clutters of Meyniel graphs are Ehrhart clutters. In much the same spirit, we provide a simple proof of our conjecture when C is a uniform clique clutter of a perfect graph. We close with a generalization of Ehrhart clutters as it relates to total dual integrality.Comment: Electronic Journal of Combinatorics, to appea

SELECTIVE LASER MELTING OF Al-Cu12 IN-SITU ALLOYING DEVELOPMENT FOR ANCHOR-LESS PROCESSING

The feedstock used within the additive manufacturing process Selective Laser Melting (SLM) is generally deposited and laser processed in a pre-alloyed state. The full melting and rapid solidification of feedstock leads to the creation of components with mechanical properties comparable and sometime exceeding those of traditionally cast. For high performance applications within aerospace and automotive, pure elemental powdered blends for use within SLM are generally not used due to poor powder distribution and difficulty in controlling composition across the component. In-situ elemental blending of feedstock represents a route for testing the feasibility of different elemental mixtures, creating alloys in-situ in a cost-efficient way, however the resultant properties of component made using such a feedstock are not fully understood. This research aims to develop an in-situ aluminium hardenable alloy using a novel Semi-Solid Processing (SSP) method known as, Anchorless Selective Laser Melting (ASLM). This method requires two or more separate materials within the feedstock to be in-situ alloyed under the action of the laser to form into various combinations of eutectic/hypo/hyper eutectic alloys in a stress reduced state. The ASLM method results in the elimination of supports required during manufacture due to maintaining the processed material in a semi-solid state. In this investigation, Selective Laser Melting (SLM) was applied to an identified suitable candidate materials for ASLM processing requiring elemental blending and developed optimum processing parameters for the in-situ fabrication of an Al-Cu12 alloy from pure elemental blends of aluminium and copper powders. Design of Experiments (DOE) were applied for parameter optimisation in order to minimise internal defects and studying the influence of SLM parameters such as layer thickness, laser power, scan strategy, scan speed and hatch spacing concluding that 67o meander scanning strategy and a combination of high power source and reduced scanning velocities leads to a higher densification. Findings shows that the use of elevated pre-heat temperatures created a coarser cellular-dendritic microstructure consisting of supersaturated Al-rich matrix with a uniform globular microstructure with finer Al2Cu phase compared to as-fabricated samples at room temperature. It was found that Al-Cu12 in-situ processed samples achieved maximum tensile strength values comparable to cast AlCu12 alloy. Processing at elevated pre-heat temperatures created components with higher ultimate tensile strength and ductility and minimised warping distortion compared to standard room temperature built samples due to it assisting a more complete melting of Al and Cu particles. An in-situ age hardening resulted of the prolonged high temperature processing and slower cool down, producing an equilibrium α + θ microstructure

Asthma referrals : a key component of asthma management that needs to be addressed

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Que lo urgente no relegue lo importante: absentismo vs. productividad

El debat sobre la productivitat dels empleats públics es barreja sovint amb al·lusions al seu grau d’absentisme. Hi ha la percepció que la productivitat és baixa i que l’absentisme és molt preocupant i és la causa principal de la primera. Tanmateix, aquesta presumpta relació causa efecte és, en molts casos, qüestionable. Per això vam voler conèixer l'opinió dels qui tenen responsabilitat política, directiva o de gestió sobre l'ocupació pública. A tal efecte vam fer una enquesta entre persones amb l'esmentat perfil a les administracions catalanes (142 enquestats). Una primera anàlisi ens permet aventurar que l'absentisme no és tan aclaparador; tanmateix, sí que preocupa la dubtosa veracitat de les motivacions de moltes de les baixes de curta durada. També destaca que la gran majoria dels entrevistats cregui que s'ha d'incidir menys en la presència efectiva de l'empleat i més en la fórmula de flexibilitat horària i d'assoliment d'objectius. Aquestes i altres observacions ens porten a qüestionar-nos si la urgència per combatre un nivell d'absentisme —al qual ni les dades ni la percepció entre gestors públics no li confereixen l'atribut d'alarmant— ens allunya del que sí que és important: la productivitat.The debate about the productivity of public employees is often commingled with allusions to the degree of their absenteeism. There is a perception that productivity is low and absenteeism is a subject of great concern and the major cause of low productivity. Nevertheless, this supposed cause and effect relationship is, in many cases, questionable. For this reason, we attempted to learn the opinion of those with political, supervisory or managerial responsibility over the public-sector workplace. To that end, we conducted a survey among people with that profile who worked in Catalan government agencies (142 people surveyed). An initial analysis allows us to put forward the thesis that absenteeism is not so overwhelming; nevertheless, the doubtful veracity of the reasons given for many of the short-term periods of sick leave is a cause for concern. Another salient point is that the large majority of those interviewed believe that efforts should be made less to influence the actual presence of the employee than to encourage flexible schedules and the achievement of objectives. These observations, along with others, lead us to question whether the urgency to combat the level of absenteeism – that neither the data nor the perception among public administrators considers alarming – distracts us from what is indeed important: productivity.El debate sobre la productividad de los empleados públicos se mezcla a menudo con alusiones al grado de absentismo de estos. Existe la percepción de que la productividad es baja y el absentismo es muy preocupante y principal causa de la primera. Sin embargo, esta presumida relación causa-efecto es, en muchos casos, cuestionable. Por ello quisimos conocer la opinión de quienes tienen responsabilidad política, directiva o de gestión sobre la ocupación pública. Para ello realizamos una encuesta entre personas con dicho perfil en las Administraciones catalanas (142 encuestados). Un primer análisis nos permite aventurar que el absentismo no es tan apabullante; sin embargo, sí que preocupa la dudosa veracidad de las motivaciones de muchas de las bajas de corta duración. También destaca que la gran mayoría de los entrevistados crea que se debe incidir menos en la presencia efectiva del empleado y más en la fórmula de flexibilidad horaria y de logro de objetivos. Estas y otras observaciones nos llevan a cuestionarnos si la urgencia por combatir un nivel de absentismo —que ni los datos ni la percepción entre gestores públicos le confieren el atributo de alarmante— nos aleja de lo que sí es importante: la productividad